1. Field of the Invention
This invention generally relates to machines for treating boards with a gaseous substance and, in particular, relates to a machine for continuously treating a board such as a particle board with a gaseous substance by drawing the gaseous substance through the board.
2. Description of the Prior Art
In our Swedish application No. 7701189-8, previously filed, it has been shown that apparently tight fiber materials have a certain permeability to gas sufficient for gaseous substances to be sucked right through the board by means of a negative pressure of the order of 0.4-0.9 negative pressure.
As is evident from the above-mentioned application, this makes possible a method of treatment for removing excessive formaldehyde from particle board by treatment with ammonia, this excess being bound in the form of hexamethylene tetramine. In treatment finally pressed boards are exposed to ammonia gas or a mixture of air and ammonia gas on one side at the same time as the opposite side of the board is exposed to a gas pressure being 40,000-90,000 Pascal lower than the pressure acting on the opposite side.
In this way, a particle board product is obtained which no longer gives off free formaldehyde, which is now a prerequisite for the use of the boards in apartments and the like due to the long-term unhealthy properties of the formaldehyde.
The negative pressure required for absorbing a certain amount of gas by time unit through a board is, of course, dependent on the permeability and thickness of the material. For certain particle boards, a negative pressure of several thousands mm water column is required. Already, one thousand mm water column gives, as is well known, a compressing strength of a thousand kg on one square meter, and, therefore, special measures must be taken to ensure a safe feeding through the suction zone.